Current limit circuit for power supply regulators

ABSTRACT

Most power supply regulators either cause the output voltage to drop-off gradually, or caused output oscillation upon reaching or exceeding the limit current. Current limit feedback to an error amplifier within a switching regulator is used to control the output voltage, prevent oscillation, insure that current limiting does not occur until a well-defined threshold is achieved and then steeply limit the current. The circuit uses feedback to the switching regulator to maintain the output voltage within desired limits. The output current flows through the current limit resistor R c  thus providing a voltage proportional to the current at the base-emitter junction of Q 1  via CR 1  and R 2 . During normal operation, the forward biased CR 1  voltage maintains a reverse bias on the B-E junction of Q 1 . When the voltage across R c  exceeds the forward bias on CR 1 , Q 1  turns on, hence turning on Q 2 , thus providing its contribution to the current limit feedback circuitry for use by the switching regulator.

FIELD OF THE INVENTION

This invention pertains to a method and apparatus for limiting currentin a power supply.

BACKGROUND OF THE INVENTION

Most prior art circuits either caused the output voltage to drop-offgradually, or caused output oscillation upon reaching or exceeding thelimit current.

OBJECTIVES OF THE INVENTION

It is therefore a primary objective of the present invention to providea circuit for regulating voltage of a power supply to provide goodregulation of voltage up to a desired current limit value.

It is another objective to drop the voltage sharply above the desiredcurrent limit.

Still another objective is to avoid oscillation of the output voltage.

A further objective is provide a highly reliable circuit.

SUMMARY OF THE INVENTION

Current limit feedback to an error amplifier within a switchingregulator is used to control the output current, prevent oscillation,insure that current limiting does not occur until a well-definedthreshold is achieved and then steeply limit the current.

These objects of the invention and other objects, features andadvantages to become apparent as the specification progresses areaccomplished by the invention according to which, briefly stated,current limit feedback to an error amplifier within a switchingregulator is used to control the output voltage and prevent oscillation.

LIST OF ADVANTAGES OF THE INVENTION

An important advantage of the present invention is that the circuit usesprovides a square voltage-current curve.

Another advantage is that the circuit uses few components.

A further advantage is that most components of the circuit are fairlyinsensitive to component tolerances or parameters.

Still another advantage is that oscillation is prevented.

These and further objectives, constructional and operationalcharacteristics, and advantages of the invention will no double be moreevident to those skilled in the art from the detailed description givenhereinafter with reference to the figures of the accompanying drawingswhich illustrate a preferred embodiment by way of non-limiting example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the circuit according to the invention.

GLOSSARY

The following is a glossary of circuit elements as referenced andemployed in the present invention.

Q₁ --2N2222A transistor

Q₂ --2N2907A transistor

Q₃ --2N3584 transistor

Q₄ --2N3584 transistor

CR1,CR2--1N414B diodes

R_(c) --0.249Ω resistor

R₁ --1000Ω resistor

R₂ --16.2Ω resistor

R₃ --4.99Ω resistor

R₄ --499Ω resistor

R₅ --3.01kΩ resistor

R₆ --499Ω resistor

R₇ --90.9Ω resistor

R₈ --10kΩ resistor

C₁, C₂ --0.01 mfd capacitors

SR--switching regulator IC, e.g. Harris FG1532

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings wherein like reference numerals are usedto designate like or corresponding parts throughout the various figuresthereof, there is shown in FIG. I the circuit according to theinvention.

The invention uses feedback to the switching regulator to maintain theoutput voltage V_(out) within desired limits. The output current flowsthrough the current limit resistor R_(c) thus providing a voltageproportional to the current at the base-emitter junction of Q₁ via CR₁and R₂. During normal operation, the forward biased CR₁ voltagemaintains a reverse bias on the B-E junction of Q₁. When the voltageacross R_(c) exceeds the forward bias on CR₁, Q₁ turns on, hence turningon Q₂, thus providing its contribution to the current limit feedbackcircuitry for use by the switching regulator SR.

R₁ is a resistor that maintains the continuous forward bias currentthrough CR₁. R₂ provides some current limiting for the base-emitterjunction of Q₁ during hard current limiting conditions and outputcurrent spikes. R₃ prevents Q₂ from being driven too hard by Q₁. R₄provides a path for leakage current from the base-collector junction ofQ₂ thus helping to hold it off during normal operation. Capacitor C₁ issimply a filter capacitor for the regulator output.

Within the feedback circuit, R₅ and R₆ scale the feedback signal to thepower control circuit. C₂ and CR2 are filters to reduce the noise to thepower control circuit.

This invention is not limited to the preferred embodiment andalternatives heretofore described, to which variations and improvementsmay be made, without departing from the scope of protection of thepresent patent and true spirit of the invention, the characteristics ofwhich are summarized in the following claims.

What is claimed is:
 1. A circuit for regulating voltage in a powersupply, comprising:a power control circuit means for controlling avoltage of a power source, a current limiting means for generating afeedback signal to said power control circuit means while limitingcurrent to an output, and a feedback circuit means for scaling a signalfrom a signal generation means to be sent to said power control circuitmeans.